threadpool: update to std::future (#1219)

An initial pass at updating `tokio-threadpool` to `std::future`. The
codebase and tests both now run using `std::future` but the wake
mechanism is not ideal. Follow up work will be required to improve on
this.

Refs: #1200

tokio-threadpool/README.md

@@ -5,43 +5,6 @@ threads.
 
 [Documentation](https://docs.rs/tokio-threadpool/0.1.14/tokio_threadpool)
 
-### Why not Rayon?
-
-Rayon is designed to handle parallelizing single computations by breaking them
-into smaller chunks. The scheduling for each individual chunk doesn't matter as
-long as the root computation completes in a timely fashion. In other words,
-Rayon does not provide any guarantees of fairness with regards to how each task
-gets scheduled.
-
-On the other hand, `tokio-threadpool` is a general purpose scheduler and
-attempts to schedule each task fairly. This is the ideal behavior when
-scheduling a set of unrelated tasks.
-
-### Why not futures-cpupool?
-
-It's 10x slower.
-
-## Examples
-
-```rust
-use tokio_threadpool::ThreadPool;
-use futures::{Future, lazy};
-use futures::sync::oneshot;
-
-pub fn main() {
-    let pool = ThreadPool::new();
-    let (tx, rx) = oneshot::channel();
-
-    pool.spawn(lazy(|| {
-        println!("Running on the pool");
-        tx.send("complete").map_err(|e| println!("send error, {}", e))
-    }));
-
-    println!("Result: {:?}", rx.wait());
-    pool.shutdown().wait().unwrap();
-}
-```
-
 ## License
 
 This project is licensed under the [MIT license](LICENSE).